A solid oxide fuel cell (referred to hereinafter merely as an SOFC where appropriate) is a fuel cell using an oxide as a solid electrolytic material having ionic conductivity. The fuel cell generally has an operating temperature as high as on the order of 1000° C., but there is lately being developed one having an operating temperature not higher than about 800° C., for example, on the order of 750° C. With the SOFC, there are disposed an anode (that is, a fuel electrode), and a cathode (that is, an air electrode or an oxygen electrode) with an electrolytic material sandwiched therebetween, thereby making up a single cell as a three-layer unit of the anode/an electrolyte/the cathode.
When the SOFC is operated, fuel is fed to the anode side of the single cell (also referred to merely as “a cell” where appropriate in the present description), air as an oxidizing agent is fed to the cathode side thereof, and electric power is obtained by connecting both the electrodes to an external load. However, with the single cell of one unit only, a voltage only on the order of 0.7V at most can be obtained, so that there is the need for connecting in series a plurality of the single cells together in order to obtain electric power for practical use. For the purpose of electrically connecting adjacent cells with each other while concurrently feeding fuel, and air to the anode, and the cathode, respectively, after properly distributing them, and subsequently, effecting emission thereof, separators (=interconnectors) and the single cells are alternately stacked.
Such an SOFC system as described above is a type wherein a plurality of the single cells are stacked one on top of another, but it is conceivable to adopt a multi-segment type in place of such a type as described, for example, in Fifth European Solid Oxide Fuel Cell forum (1-5, Jul. 2002) p. 1075-, the external appearance of the multi-segment type and so forth are disclosed although the contents thereof are not necessarily clear-cut in detail. As the multi-segment type, two types including a cylindrical type and a hollow flat type are conceivable.
FIG. 1 is a view showing an example of the structure of the hollow flat type of the two types, FIG. 1(a) being a perspective squint view, FIG. 1(b) a plan view, and FIG. 1(c) a sectional view taken on line A-A in FIG. 1(b). As shown in FIGS. 1(a)-(c), there are formed a plurality of cells 2 each made up by stacking an anode 3, an electrolyte 4, and a cathode 5 in that order on an insulator substrate 1 in a hollow flat sectional shape, and the respective cells 2 are structured so as to be electrically connected in series with each other through the intermediary of an interconnector 6, respectively. Fuel is caused to flow in space within the insulator substrate 1, that is, an internal fuel flow part 7, in parallel with a lineup of the cells 2, as indicated by an arrow (→) in FIGS. 1(a) and 1(c).
Now, with the SOFC system of the hollow flat type as described above, the fuel is going to become thinner as it moves in the direction of its flow. Nevertheless, since the respective cells are disposed so as to be electrically connected in series, the same current is forced to flow between the cells even under thinned fuel. Consequently, voltage drop increases, thereby causing a problem of lower power generation efficiency. In addition, since the respective cells are connected in series in one direction, a voltage obtained is limited.